i40e: little endian only valid checksums

The calculation of the checksum can fail.
So move converting the checksum to little endian
to inside the return status check.

Signed-off-by: Tom Rix <trix@redhat.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>